NEW ORLEANS, Sept. 17 -- Business owners trickled into New Orleans on Saturday, poking through glass shards and musty offices as the head of the federal relief effort warned in the strongest terms yet that the city is still unsafe for the 180,000 people being invited to return this week by Mayor C. Ray Nagin.

The dire assessment by Vice Adm. Thad W. Allen, appointed by President Bush to oversee the Federal Emergency Management Agency's recovery program, places him at odds with a mayor under increasing pressure from the business community to demonstrate New Orleans is on the mend.

"The return of the general population to the city of New Orleans is problematic," Allen, the Coast Guard's chief of staff, said in an interview. The mayor announced on Thursday a phased-in return beginning this weekend with businesses followed soon by three neighborhoods with minimal damage.

Water, sewage and electrical systems are unable to "meet the basic needs of the businesses and residents who return," Allen said in a statement. He urged people "to use extreme caution if they return and to consider delaying their return until safer and more livable conditions are established."

In St. Bernard Parish, officials allowed small groups to return Saturday to retrieve belongings, though many left empty-handed. Authorities estimated that more than half of the 25,000 houses in the parish are damaged beyond repair. "They talk about rebuilding all these homes, but there's no way," said George Ansardi, a general contractor, outside his destroyed house.

Allen is concerned about myriad unresolved health, safety and environmental dangers that could worsen as thousands descend on a city where entire neighborhoods are uninhabitable and cleanup crews have donned masks, rubber gloves and white body suits to protect themselves.

"You need to address these issues and reduce risks before the general population can be let back into the city," he said.

Although Allen acknowledged that the decision to reopen the city ultimately lies with the mayor, he plans to meet with Nagin on Monday to discuss his fears directly. Nagin was visiting his family Saturday in their relocated home in Dallas. In a briefing Friday, his homeland security director and the city attorney acknowledged the poor conditions and lack of basic services, and warned that people entered New Orleans at their own risk.

Allen's unusual decision to openly challenge Nagin's plan to allow thousands of people to return to New Orleans was reminiscent of the early political squabbling of the Hurricane Katrina disaster. Even now, as the crisis approaches the three-week mark, state and federal leaders often hold simultaneous, dueling news conferences in Baton Rouge, and some have said they are not in contact with Nagin and do not even know the phone numbers of the mayor or his top aides.

On the first day business owners were officially allowed to return, the anticipated mass traffic jams and checkpoint quagmires never materialized. After vowing to conduct detailed checks -- requiring documentation proving that people who returned to the city owned business -- New Orleans seemed to forget its resolve. Many entrances to the city were left unguarded, and cars breezed along streets that still lack working traffic signals. At some checkpoints, the arriving business owners were better informed than the troops guarding the entrances.

Sean Wallace, a mixed-media artist, encountered a stupefied guard on his way in from northwest of the city. "He asked me, 'Why are you coming in?' " Wallace said. "It was the first he'd heard about businesses being let back in."

Wallace, his artist's eye attuned to light and color, encountered a deflating scene of trees stripped raw and piled branches. "New Orleans was always my favorite city to return to -- no matter where you go, New Orleans is always greener and lusher," he said. "There's too much sunlight hitting the ground; everything is brown."

As clipboard-toting insurance adjusters stepped over debris, Salvation Army volunteers distributed sandwiches, bottles of water, and franks and beans. New mattresses arrived at the Crowne Plaza Hotel on Canal Street, and Robby Crain hauled crates of spoiled food out of his Subway sandwich shop on Royal Street.

With horrifying images of the hurricane aftermath beamed around the world, Nagin acknowledged Thursday that he was getting pressure to move more quickly. He ebulliently announced a plan to let as many as 180,000 people in within a week, saying, "We're bringing New Orleans back. It's a good day in New Orleans. The sun is shining."

Stepping up the timetable played well in the French Quarter, where jeweler Franco Valobora said, "As I've been telling the mayor, New Orleans is a dying patient, and we need to give it a boost to the heart and the heart is here in the French Quarter."

Tiring of outsiders, some New Orleanians spoke confidently about celebrating Mardi Gras here next February and said it was time for out-of-town rescue workers and politicians to leave and let residents take over.

"The day the president came, I couldn't get contractors in," said Wayne Cox, assistant superintendent of engineering services for Pan American Real Estate. "We've got gaping holes in the roof, but nobody can get through because Mr. Bush wanted to come."

Old-time New Orleans meshed Saturday with the modern, wrecked New Orleans on the quiet boulevards and spooky streets that people encountered on their return. Philip Collier -- brave or, perhaps, foolish -- climbed into an elevator and ascended three floors in the Factor's Row Building, where Edgar Degas painted his American masterpiece "The Cotton Exchange." Collier, a graphic designer whose big, bold images grace posters for the New Orleans Museum of Art and the Ogden Museum of Southern Art, was in search of something irreplaceable: the family photo albums he had stashed in office storage space. "I love this city," Collier said. "It's totally heartbreaking."

Like many entrepreneurs here, Collier is employing a good bit of creativity to survive. He plans to operate his business through cell phone and Internet connections with designers living in Covington and Avery Island.

Collier will also take care of unfinished business. A book he compiled before the storm was supposed to have been published Aug. 29, the day Katrina hit, and his editors are more eager than ever to see it print. The title: "Missing New Orleans."

Staff writers Keith L. Alexander in New Orleans and Peter Whoriskey in St. Bernard Parish contributed to this report.